Abstract : The ventilation design criteria for both road and rail tunnel is based on the design fire defined by the standards and the general knowledge about smoke propagation. The problem of such an approach is that it considers only the impact on the safety ventilation of the smoke propagation and dispersion inside the tunnel excluding other possible accident. However some other situations, such as toxic gas release, are possible and even if the aim is not to design the ventilation on other dangerous phenomena with a lower occurrence frequency, it must be ensure that the ventilation system does not increase the consequences of the accident. Mainly, the problem of toxic gas dispersion is pointed out in this paper. Because of the large variety of dangerous materials that can transit in tunnel, the probability of an accident that impacts a toxic transport cannot be neglected. In the worst case scenario, such as a massive release of high toxic gases, the ventilation is useless because of the toxic quantity that induces a large number of deaths inside the tunnel. However, when the toxic release is lower and ventilation can be used, having in mind that toxic gas is generally heavy gas or a cold gas, the behaviour will of course be different than the one of smoke and the ventilation system may not be adapted for such a situation. This case has scarcely been studied yet. In this study, both experimental approach and numerical tools were used to improve the global understanding of dense gas dispersion in underground infrastructure such as road tunnels. The experimental work was achieved in the INERIS fire gallery which represents a 50 m long 1/3rd scale tunnel using Argon. It was achieved for different leaks conditions in order to appreciate the dense gas natural behaviour. This work has also enabled the comparison between experimental work and CFD calculation with FDS code for the particular application of dense gas dispersion. . The work was extended to some other configurations and geometry in order to simulate real scale situation with different kind of gases : a highly toxic dense gas such as Chlorine, a light gas stored as a liquid at a very low temperature such as Ammonia, and a gas which remains liquid at ambient temperature and pressure and is drained into an evaporating pool such as Acrolein. This work will consider the natural behaviour of the gases and the influence of longitudinal ventilation both inside and outside of the tunnel